That phrase is from the Declaration of Independence, one of the US's founding documents:

"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."

http://www.archives.gov/exhibits/charters/declaration_transc...

The nature of self-evidence is that you don't have to say how.

(Though I'm not sure I agree with the parent point that Americans as a group really believe equality in the relevant sense here.)